Falling gasoline prices have sparked congressional debate about increasing the federal gas tax to help fund upcoming infrastructure projects and have set up a potential disagreement among House and Senate Republicans.

The federal gas tax primarily pays for transportation projects but has stayed at 18.4 cents a gallon for roughly two decades, helping create the funding gap.

In addition, increasing construction costs combined with less revenue from the gas tax, in large part because of more fuel-efficient vehicles, have further contributed to the estimated $16 billion funding gap over the past several years.

Though Congress has managed to find money elsewhere in the federal budget to cover the shortages, lawmakers say the country needs more than a stop-gap solution.

However, House Speaker John Boehner suggested Thursday that getting a gas-tax increase passed in the now-Republican-controlled House and Senate seems unlikely.

“When the Democrats had total control of the Congress they couldn't find the votes,” he told reporters. “It's doubtful the votes are here to raise the gas tax again. … I’ve never voted to raise the gas tax. We'll have to work our way through it.”

But at least four Senate Republicans -- Bob Corker, Tenn.; James Inhofe, Okla.; Orrin Hatch, Utah; and John Thune, S.D. -- appear open to the idea of increasing the tax.

Last week, Thune, the third-ranking Senate Republican, told “Fox News Sunday” that he didn’t “favor increasing any tax, but I think we have to look at all of the option.”

Gas prices are now $2.50 a gallon, which some economic experts argue now gives Americans enough money to absorb such a tax increase.

The federal highway bill expires at the end of May. And there is a roughly $100 billion shortfall on funding the agency’s Highway Trust Fund at its current levels.

Inhofe, chairman of the Senate Committee on Environment and Public Works, has suggested lawmakers have little choice but to at least consider an increase, in light of the state of the fund and the country’s crumbling roads and bridges.

Corker, who is backing a proposal for a 12-cent-a-gallon increase in the gas tax over the next two years, says such a hike would be offset by other taxes that Americans now pay.

“At least it would put our infrastructure on strong footing,” he said. “And that second component seems to get left out of the conversation most of the time. But, yes, I believe that's what we should do.”

Democrats including Sen. Barbara Boxer and House Minority Leader Nancy Pelosi, both of California, have urged Congress to increase the tax or find other ways to better fund infrastructure projects.

“If there's ever going to be a time to raise the gas tax, the time when gas is so low is the time to do it,” Pelosi said last week.